Fix indentation
Use rtSMC_Scriptable as the mruby internal check type.

This way we don’t have to rely on mruby’s DATA_PTR to get the
C++ object out of the mruby one, but can use mruby’s own proper
methods. Additionally, mruby’s typechecking isn’t entirely
senseless anymore (it would now detect non-SMC-C++ objects, if
there were any).
Fix bug in UID assignment from XML files.

Previously UIDs loaded from XML files were not marked as taken
from the pool of available UIDs.